Various configurations have been proposed to guide and detect a catheter probe through the internal spaces of a patient undergoing a surgical procedure. These proposed configurations are characterized by several alternative approaches including, inter alia, procedures for solving equations to determine unknown location parameters, the generation and detection of magnetic fields, and the use of sensing devices affixed to the catheter probe.
U.S. Pat. No. 4,905,698 to Strohl, Jr. et al. discloses a locator device external to a subject for generating an electromagnetic field that projects into the subject. A catheter inserted into the subject is fitted with a sensing coil at its distal end. The phase of the voltage that is induced in the coil in response to the field is compared to the phase of the generated field. When an in-phase condition occurs, this is an indication that the locator is behind the coil; alternatively, an out-of-phase condition indicates that the locator is beyond the coil. Positions intermediate these two rough approximations of the coil position are not determined other than by a beeping indicator that signifies that this intermediate positioning has been reached.
U.S. Pat. No. 4,821,731 to Martinelli et al. discloses an electroacoustical transducer means secured to the distal end of a catheter that is inserted into a subject for generating acoustical pulses that propagate along an imaging axis and reflect from an anatomical area of interest. The acoustic echoes are converted by the transducer means into electrical signals representative of an image of the anatomical area under reflection and the relative position of the transducer means and angular orientation of the sensing/imaging axis.
U.S. Pat. No. 4,642,786 to Hansen discloses a magnetic position and orientation measurement system that determines the location of an object in space with various configurations, each characterized by the attachment of a retransmitter to the object consisting of passive resonant circuits. The retransmitter is in a predetermined position and orientation with respect to the object. A magnetic field is generated at a resonant frequency of the retransmitter which then retransmits a magnetic field for subsequent reception. The position and orientation of the object may be calculated based upon the induced signals as developed by the reception of the retransmitted magnetic field. The original transmission and reception may be implemented with an integrated transceiver, separate transmitter and receiver elements, or a single transmitter and an array of receiver coils.
U.S. Pat. No. 4,317,078 to Weed et al. discloses how the location of a magnetically sensitive element may be determined by moving a magnetic field source along specified reference axes to induce signals in the sensor so as to identify a set of null points representative of certain flux linkage values. The null point locations are used to calculate the sensor position.
U.S. Pat. No. 3,868,565 describes a system where a magnetic field is generated which rotates about a known pointing vector. The generated field is sensed along at least two axes by a sensor attached to the object to be located or tracked. Based upon the relationship between the sensed magnetic field components, the position of the object relative to the pointing vector can be computed.
U.S. Pat. No. 4,173,228 to Van Steenwyk et al. discloses a catheter locating system that includes a sensor attached to the distal end of the catheter. An electromagnetic field is projected into the body cavity with magnetic probe coils. The field is detected by the sensor, which generates an induced signal whose magnitude and phase are representative of field strength, separation of sensor and probe coils, and relative orientation of sensor and probe coils. The probe coil undergoes linear and rotational movement to identify orientations and locations of the probe coil where minima and maxima occur in the measured signal induced in the sensor. This information is representative of the position and orientation of the sensor.
U.S. Pat. No. 5,211,165 to Dumoulin et al. discloses a modified catheter device that includes a small RF transmit coil attached to its distal end. The transmit coil is driven by an RF source to create an electromagnetic field that induces electrical signals in an array of receive coils distributed around a region of interest. Alternatively, the receive coils can be placed on the invasive device and the transmit coils are distributed outside the patient. A minimum of one transmit coil and three receive coils is necessary to precisely determine the location of the invasive device. A series of equations is developed to solve for the unknowns x-y-z-.phi.-.theta..
PCT Application No. WO94/04938 to Bladen et al. describes how the location and orientation of a single sensing coil may be determined from induced signals developed in response to a sequence of applied magnetic fields emanating from three groups of field generators each including three mutually orthogonal coils.
The positioning methodology developed by Bladen et al. involves calculating the distance from the sensing coil to each group of field generators as a function of the induced voltage developed in the sensing coil by the field generator. The distance calculation is used to define the radius of a sphere centered on the respective field generator. The intersection (i.e., overlap) of the spheres is used to calculate an estimate of the sensor position, using the spherical radius extending from the known location of the field generators as the estimate for each generator.
The orientation algorithm of Bladen et al. develops general equations for induced voltage including the entire set of unknown variables (x-y-z location and .phi.-.theta. orientation). The algorithm specifically solves for the orientation parameters by substituting the measured induced voltage and the computed x-y-z coordinates into the general induced voltage equation, and then reduces the equations to the unknown variables .phi.-.theta..
In an alternative orientation algorithm described by Bladen et al., the induced voltage is treated as a vector quantity, allowing the angle between the magnetic field at the generator and the radial vector joining the sensor to the generator to be calculated with a dot product computation. The angle between the radial vector and the sensor axis can be determined from the computed field angle using the dipole equations that define the generator fields. This sensor angle and the radial position as determined by the position algorithm together define the sensor position for use in the alternative orientation algorithm. These values are used to compute the angular orientation .phi. and .theta..